1. Field of the Invention
The present invention relates to a color conversion in which color image signals or color video signals are inputted and transformed into desirable colors, for example, utilized for a color hard copy apparatus, a color display apparatus, a color TV camera apparatus, a color recognition apparatus, a video editing apparatus and the like.
2. Description of the Prior Art
Conventionally, in an image processing for monochrome images, one picture plane of an image has only one-dimensional information; i.e. lightness. Therefore, in order to perform a lightness conversion as a gamma curve conversion, a real-time color conversion can be realized by memorizing various non-linear curves into Look-Up Tables (LUT).
Furthermore, even in the case where color images are dealt with in such an image processing, in order to realize a real-time color conversion, three pieces of monochrome image plane consisting of a Red plane, a Green plane, and a Blue plane were utilized. And, a conversion processing of each monochrome color plane was independently executed by means of an exclusively provided LUT.
However, the color conversion actually handled in such a conventional processing is essentially the same as one-dimensional processing. That is, the color conversion in accordance with this type of image processing is normally expressed as follows: EQU R'=hR(R), G'=hG(G), B'=hB(B)
In a color image processing, since information included in one picture plane is a three-dimensional information of (R, G, B), it will be understood from the view point of a substantial meaning of the color image processing that the color conversion should be defined as follows: EQU R'=fR(R,G,B), G'=fG(R,G,B), B'=fB(R,G,B)
For example, in a color image processing operation, it is required to accurately reproduce colors by transferring images on one color display device to other color display device or to accurately reproduce manuscripts of color photographs or color printed matters on a color hard copy. Furthermore, there is a complicated requirement for color adjustment or color conversion such that only specific color is required to be adjusted for accomplishing a color design. Such a processing is involved in the processing using above-described function comprising three variables.
For example, as one of techniques becoming important in a recent color image processing art, there is known an HLS-transformation in which a color expressed in a form of (R,G,B) is transformed into a hue H, a lightness L, and a saturation S.
According to this technique, one input is defined as a function of three inputs such as H=H(R, G, B). Therefore, it can be concluded that this type of color conversion belongs to above-described three-dimensional conversion.
In the case where such a color conversion is carried out by use of a conventional memory table, however, if one color is defined as an 8-bit signal, a large memory capacity up to 16M byte would be required for realizing only one color conversion. Accordingly, an appropriate hardware capable of being widely used and realizing a real-time processing was required in the conventional art for executing certain color conversion in compliance with such a three-dimensional color conversion.
However, in the case where such a processing is executed by using hardware in order to realize a real-time processing, it was difficult to make a numerical formula model since various color devices are associated. Otherwise, even if the numerical formula model could be produced, a hardware used for realizing this numerical formula becomes complicated and special.
For such reasons, there have been used a three-dimensional color look-up table capable of renewing data, which has inputs of dispersed lattice points disposed in a three-dimensional space defined by three variables such as RGB, and a three-dimensional interpolation means for compensating an input between dispersed lattice points.
For example, there has been known a color signal interpolation method (For example, refer to Japanese Patent No. SHO 58-16180) which mainly executes a color correction for a color hard copy and a color scanner by dissecting an input color space into a plurality of color spaces, selecting a plurality of color correction information of their vertices, and carrying out a weighting processing to output an interpolated result.
This prior art discloses an idea for simplifying the interpolation calculation in such a manner that, in the interpolation processing, a unit cube is set in the three-dimensional color signal space, and this unit cube is dissected into a plurality of tetrahedrons, and further the interpolation calculation is executed on the basis of output signals obtained from respective vertices of the tetrahedrons.
By utilizing this method as a color conversion apparatus capable of being widely used, a non-linear flexible color conversion such as a color change of a specific color in the color space can be carried out at a high speed while a gradation of image is maintained without being changed.
However, in the case where the input color space includes regular three-dimensional lattice points forming unit interpolation sections, the input color space in accordance with the conventional art was dependent on properties of device such as a three-principal-color reflectance, a transmittance, or a three-principal-color density derived from each color scanner. And, its three-dimensional coordinate axes are respectively divided by being uniformly calibrated.
This shows that the conventional technique has a problem in that it does not utilizes an important human property; i.e. the human visual characteristics on one hand requires a fine gradation in a lightness direction but on the other hand does not require so fine gradation in a chromaticity direction.
Furthermore, since the conventional technique sets a tetrahedron as a unit interpolation section which is formed by dissecting a unit cube in the input color space into a plurality of tetrahedrons, in the case where the input color changes along a line parallel to the lightness direction, the input color passes a plurality of different tetrahedrons and is interpolated linearly in each tetrahedron independent of one another.
For this reason, when the color conversion is a non-linear conversion, there was a problem such that a certain input having a gradation in a direction of gray color, which is most important in the human visual characteristics, cannot have a linear and smooth interpolation output value, and rather has an unnatural interpolated output value such as an unnaturally broken line.
Still further, in case of the conventional technique, when required to change output values at lattice points of the color space for color adjustment and so on, there was a problem such that a space utilized for color conversion and a space utilized for interpolation do not coincide with each other.
This is because the color space including lattice points used for interpolation is chiefly an RGB space or a density space, on the other hand, the space convenient for color conversion is often a separation space of lightness and chromaticity suitable for human color vision or a polar coordinates of chromaticity, saturation, and lightness defined in a lightness-chromaticity space. That is, the lattice points serving as color control points are not regularly arrayed in the color space executing color conversion. In some cases, even if a specific color region is designated, a corresponding lattice point may not actually exist.